1. Field of the Invention
This invention relates to controlling apparatus for controlling the speed of a wound rotor type induction motor below and above a synchronous speed thereof.
2. Description of the Prior Art
Recently, there has been constructed a super-synchronous Scherbius apparatus which enables wound rotor type induction motors to have a smooth speed control below and above the synchronous speed by controlling the secondary power in either the forward or backward direction through the connection of a cyclo-converter on the secondary side.
A prior Scherbius apparatus in which the rotor winding is connected to a rectifier comprising a number of diodes does not generate a driving torque at and above the synchronous speed because the secondary current for driving is not generated.
It is known that an induction motor generates a torque obtained by multiplying the induced flux of the rotating magnetic field by the secondary armature current.
If the current can be generated in a winding of the rotor corresponding to the position of the rotating magnetic field, the induction motor can generate the torque even at the synchronous speed.
During such conditions, if the current is larger than the load current, the speed of the induction motor exceeds the synchronous speed.
At that time, the polarity of the secondary armature induced voltage with respect to the position of the rotating magnetic field is reversed, but the current must be set at the same polarity below the synchronous speed in order to rotate the induction motor in the same direction.
Thus, despite speeds below and above the synchronous speed, the super-synchronous Scherbius apparatus generates the torque in the same rotating direction when current of the same polarity as above mentioned is applied to the coil corresponding to the position of the rotating magnetic field and generates a reverse torque when current of reverse polarity is applied. The torque in the same rotating direction acts as a driving torque and the torque in the reverse rotating direction acts as a braking torque.
Accordingly, in the super-synchronous Scherbius apparatus, it is necessary to detect the relative position of the rotor coil with respect to the rotating magnetic field. The relative position can be directly detected by the secondary voltage induced in the rotor.
However, as is known, since the secondary voltage proportional to the slip becomes zero or a very small value at or close to the synchronous speed, the use of the secondary voltage as the detecting signal is improper.